1. Field of the Invention
Example embodiments of the present invention relate to a method of reading data from semiconductor memory devices. More particularly, example embodiments of the present invention relate to reading data in non-volatile semiconductor memory devices using different read voltages.
2. Description of the Related Art
A flash memory device is one type of non-volatile semiconductor memory device, which continuously maintains stored data. Data may be electronically programmed in or erased from flash memory devices. Further, flash memory devices may not require refresh operations. A flash memory device may be classified according to its structure as a NAND type flash memory device and a NOR type flash memory device.
In a NOR type flash memory device, a ground voltage VSS may be applied to a source region of a transistor in a memory cell to program or erase data in any address. The NOR type flash memory device typically has a relatively fast response speed. In a NAND type flash memory device, cell transistors are electrically connected to one another in serial to form unit strings, electrically connected to a source region and a drain region. The NAND type flash memory device typically has a higher degree of integration.
FIG. 1 is a circuit diagram illustrating a cell string of a NAND type flash memory device.
Referring to FIG. 1, the NAND type flash memory device includes a string select transistor SST and a ground select transistor GST. The memory cells are electrically connected to one another in serial between the string select transistor SST and the ground select transistor GST. One cell string may include 32 memory cells MC in the NAND type flash memory device, as illustrated in FIG. 1.
Each of the memory cells may be controlled by one word line, and the memory cells controlled by one word line may form one group of memory cells. Conventionally, reading data such as “0” or “1” may be performed by confirming a threshold voltage of the transistor in the memory cells.
For example, about 0V may be applied to a word line electrically connected to a selected cell, and a pass voltage for turning on non-selected cells may be applied to word lines electrically connected to non-selected cells regardless of data stored in the non-selected cells. The pass voltage may be applied to a string select line SSL and a ground select line GSL to turn on each of the transistors electrically connected to the string select line SSL and the ground select line GSL. After a voltage is applied to a bit line electrically connected to the selected cell, current in the selected cell may be confirmed. When the current is substantially higher than a standard current, it is confirmed that the selected cell is turned on. When the current is substantially lower than the standard current, it is confirmed that the selected cell is turned off. Generally, a threshold voltage of memory cells in a turned-on state may be substantially lower than about −3V, and a threshold voltage of memory cells in a turned-off state may be substantially higher than about 1V. Programmed cells may have a threshold voltage of about 1V.
However, as the degree of integration increases, distances between word lines decrease. This generates RC delay, coupling noise and/or disturbance between adjacent word lines, for example.
A voltage difference between the word line electrically connected to the selected cell and the word lines electrically connected to the non-selected cells adjacent to the selected cell may increase in a reading operation. Thus, the threshold voltage of the selected cell may be lowered considerably by the voltage difference. That is, a strong fringe field may be generated between the selected cell and the non-selected cells adjacent to the selected cell, lowering the threshold voltage of the selected cell.